1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunctional machine having several of these functions, etc.
2. Description of the Background Art
A conventional image forming apparatus generally employs a developing process for visualizing a latent image formed on a photoconductive member.
In the developing process, two-component developer including carrier and toner attracted to the carrier, or one-component developer only including toner, is utilized. Typically, the developer is supplied to a latent image by a contact system that makes the developer contact and attracts the toner to the surface of the photoconductive member by an electrostatic force at a gap formed between the latent image and the developer. Alternatively, a non-contact system that makes toner soar in an electric field generated by a developing bias and attracts the toner to the latent image is known.
In such a developing process, when the toner having a prescribed amount of charge and the latent image having a prescribed electrostatic force are balanced electrostatically excess toner detaches from the latent image and is not used but collected by the developing device as unused toner.
Such collected and unused toner sometimes causes problems. For example, the toner is charged triboelectrically because it is stirred and mixed in the developing device before being supplied to the latent image, or subjected to friction at a gap between a surface of the developing roller arranged facing a latent image carrier and a blade arranged approximating the surface of the developing roller in the developing device.
In particular, in a developing system using one-component developer, a toner layer is uniformly made into a thin layer on a surface of a developing roller, such as a metal roller, etc., and causes toner to soar and reach a latent image. In such a situation, due to friction caused by the blade for thinning the toner layer and that caused between toner particles on the surface of the developer roller, as well as stress generated by stirring and mixing thereof in conveyance and stirring steps, performance of the toner, such as charge performance, powder performance, etc., sometimes degrades.
The deterioration of toner, especially that of the performance of powder toner, is probably caused by external additives. More specifically, when fine-powder external additives, such as hydrophobic silica, titan oxide, alumina, fatty acid metal, etc., which is included in the developer to adjust fluidity, charge amount, or cleaning performance, receives friction and stirring stress, these additives bury into the toner as time elapses.
Toner with such external additives increasingly generates an intensive attraction, and adversely affects a transfer process. Since the degraded toner exerts an intensive attraction on a photoconductive member or an intermediate transfer member, the toner is hardly transferred. In particular, the toner is barely transferred onto concavities in the surface of the paper even if a transfer electric field is applied as appropriate. Accordingly, such toner heavily affects the transfer process for a sheet having a low surface smoothness, and as a result, image quality is significantly degraded.
Further, such toner also provokes toner scattering and background staining. The toner is replenished to appropriately adjust density of the developer as developer density decreases. However, when fresh toner is introduced into a developing device and is mixed with degraded toner remaining therein (hereinafter, referred to as degraded toner), the degraded toner is charged to an opposite polarity due to friction therebetween. As a result, the developing roller cannot hold the degraded toner, causing toner scattering. At same time, the degraded toner is attracted by a voltage of the background of the photoconductive member, and stains the background thereof.
Conventionally, to prevent the above-mentioned problem caused by degraded toner, a method of replacing toner on a prescribed condition, or that of forcibly consuming toner by forming and collecting a consumption pattern with a cleaner for cleaning a photoconductive member or an intermediate transfer belt per image formation, has been proposed as described in Japanese Patent No. 3171345, and Japanese Patent Application Laid Open Nos. 2007-108623 and 2008-261935 (JP-2007-108623-A and JP-2008-261935-A, respectively).
However, depending on a type of a sheet used for image formation, toner replenishment is not necessarily executed, because to do so that toner is wasted and thereby running cost increases.